1. Field of the Invention
The present invention relates to an electric power converter suitable for a DC/DC converter, particularly, which steps up a direct voltage at an optional magnification of one to two times or more and steps down the direct voltage at an optional magnification of one time or less.
2. Description of the Related Art
An electric power converter includes a DC/DC converter which converts DC (direct current) voltage. The DC/DC converter is used in various devices, for example, a power generator using a solar cell, a wind power generator, a fuel cell system, a hybrid vehicle and the like. In particular, if the DC/DC converter is applied in a technological domain, wherein there is severe, spatial and weight limitations, particularly in a motor vehicle, more demands for downsizing and weight reduction of the DC/DC converter are generated.
As for a conventional step-up DC/DC converter, for example, in a step-up DC/DC converter circuit disclosed in Patent Document 1 JP 2006-271101A, a switch is alternately turned ON/OFF. When the switch is turned ON, magnetic energy is accumulated in an inductor. When the switch is turned OFF, the magnetic energy accumulated in the inductor is supplied to an output part as electric power. In this time, as an output voltage from the inductor adds to a power supply voltage, a step-up voltage in the aggregate is obtained at the output part. A step-up ratio, namely, a ratio of an output voltage to an input voltage, changes depending on an ON-time duty ratio of the switch.
However, in this step-up method, an inductor having a heavy, large core is necessary to prevent magnetic saturation of the inductor and step up voltage sufficiently. This constitutes a factor of impediment to downsizing and weight reduction of an entire DC/DC converter.
For this reason, as for devices such as a cellular phone and the like, wherein there is a great need for downsizing and weight reduction, a charge pump circuit used a capacitor called as a flying capacitor is recommended as a voltage conversion mode (for example, refer to Patent Document 2 JP 2003-61339A). Similarly, as in the past, a stabilized power supply circuit used a switched capacitance mode is also recommended as usual (for example, refer to Patent Document 3 JP 2003-111388A).
In these modes, it is general to use the flying capacitor for storing electric power. Charging for a plurality of flying capacitors is alternately repeated by means of alternate switching of a plurality of switches and the like. This method outputs a fixed voltage of twice times but fails to flexibly select and output an optional ratio of conversion in accordance with necessity.
In this context, in a conventional configuration and method shown in FIG. 14 and FIG. 15, a step-up and step-down DC/DC converter with an optional step-up ratio of one to two times or with a step-down ratio of one time or less is suggested (refer to Patent Document 4 JP 2005-224060A). Switches in the DC/DC converter shown in FIG. 14, from a first switch SW 1 to a fourth switch SW4, respectively serve as an element of IGBT part 1 which includes a flywheel diode D1 and D2 and of IGBT part 2 which includes a flywheel diode D3 and D4. This DC/DC converter selectively makes functional an inductor L and a plurality of capacitors, C1 and C2, by making ON/OFF control of four switches, from SW1 to SW4, according to mode of operation and operates in any mode of operation out of step-up, continuity, and regeneration (step-down).
Step-up mode will be explained. First, refer to FIGS. 14A and 14B, a charging operation of capacitor C2 is explained. At a time t1, when a gate voltage is applied to switch SW2 in IGBT part 1 and subsequently the switch is turned ON (other switches, the switch SW1, the switch SW3 and the switch SW4, are all OFF), a charging current I1 flows through a route of a power supply E, an inductor L, the switch SW2, a capacitor C2, a flywheel diode D4 and the power supply E. This time, the capacitor C2 is charged by the power supply E (I1 in FIG. 14B: refer to C2 charging current waveform). Then, magnetic energy is accumulated in the inductor L. Simultaneously, as the capacitor C1 and C2 are connected to the capacitor C3 and a load R, the capacitor C3 is charged and an output current flows through the load R.
Next, at a time t2, the switch SW2 is turned OFF (other switches, the switch SW1, the switch SW3 and the switch SW 4, are all OFF), a charging current I2 flows through a route of the inductor L, a flywheel diode D1, the capacitor C1, the capacitor C2, the flywheel diode D4 and the power supply E (I2 of FIG. 14B: refer to L flywheel current waveform).
Consequently, a charging operation of the capacitor C1 will be explained by referring to FIGS. 14C and 14D. When a gate voltage is applied to the switch SW3 in the IGBT part 2 and the switch SW3 is turned ON, (other switches, the switch SW1, the switch SW2 and the switch SW4, are all OFF) a charging current I3 flows through a route of the power supply E, the inductor L, the flywheel diode D1, the capacitor C1, the switch SW3 and the power supply E. This time, the capacitor C1 is charged by the power supply (I3 of FIG. 14D: refer to C1 flywheel current waveform). Then, magnetic energy is accumulated in the inductor L. Then, simultaneously, as the capacitor C1 and C2 are connected to the capacitor C3 and the load R, the capacitor C3 is charged and the output current flows through the load R.
Next, at a time t4, when the switch SW3 is turned OFF (other switches, the switch SW1, the switch SW2 and the switch SW4, are all OFF), a charging current I4 brought by the magnetic energy accumulated in the inductor L flows through a route of the inductor L, the flywheel diode D1, the capacitor C1, the capacitor C2, the flywheel diode D4, and the power supply E (I4 of FIG. 14D: refer to L flywheel current waveform).
In this context, a step-up operation can be carried out by alternately flowing the charging current (I1 and I3) from the capacitor C1 and C2 to the power supply E, accumulating the magnetic energy in the inductor L with the charging a current (I1 and 13) into the capacitor C1 and C2, and charging the capacitor C1 and C2 with the flywheel current (I2 and 14) of the inductor L.
In this conventional DC/DC converter, as shown in FIG. 14E, an ON-time duty ratio of each switch SW2 and SW3 is assumed to be 0 to 50 percent or less. That is, considering dead time to avoid short circuit between the switch SW2 and the switch SW3, these switches are turned ON/OFF while a duty ratio is 50 percent or less, for example, with a duty ratio of 45 percent or the like. This enables an input voltage to be output at an optional step-up ratio of one to two times.
Next, an operation of step-down mode (regenerative mode) as to a conventional DC/DC converter will be explained. For example, when a motor and the like is used as a load on an output side, a cycle of the motor is controlled to decelerate (regenerative braking operation), a voltage on the output (load) side is increased, a power supply such as a battery and the like on the input side can be charged by stepping down the voltage on the output (load) side (by returning energy to the input side).
FIG. 15 shows an operation when a step-down ratio is low (approximately from 0.8 to 1 times, a regenerative load is light). when a step-down ratio is low (approximately from 0.8 to 1 times), for example, a voltage ratio applied to a regenerative power supply Eg (a voltage at the output part) and to a load Rg (a power supply of DC power supply input part) shown in FIG. 15 (a) is approximately from 1:0.8 to 1:1, a step-down operation is carried out by making ON/OFF control of the switch SW1 only, while the switch SW2 and the switch SW3 are turned OFF and the switch SW4 is ON at all the times.
A step-down operation of voltage as to the conventional DC/DC converter will be explained. First, at a time t1 shown in FIG. 15B, when the switch SW1 is turned ON, a charging current I1 flows through a route of the capacitor C1, the switch SW1, the inductor L, the capacitor C4, the switch SW4, and the capacitor C2. The load Rg (a power supply of DC power supply input part) is charged as the capacitor C4 is parallel-connected with the load. Further, this time, magnetic energy is accumulated in the inductor L in FIG. 15B: refer to charging current waveform of C1 and C2).
Next, at a time t2, as the switch SW1 is turned OFF, a charging current I2 brought by the magnetic energy accumulated in the inductor L flows through a route of the inductor L, the capacitor C4, the flywheel diode D3, and the flywheel diode D2 (I2 in FIG. 15B: refer to L charging current waveform).
In this context, when a step-down ratio is low (approximately from 0.8 to 1 times, a regenerative load is light), regeneration is carried out only by making ON/OFF control of the switch SW1. Subsequently, as shown at the lowest bottom of FIG. 15B, only when an electric charge is released from the capacitor C1 and C2, which are connected in series, and only when energy accumulation of the inductor L is released, an output current iL flows through the capacitor C4 on the input side. As a result, the output current iL is turned into an interrupted current and is eventually interrupted. Then, in this conventional example, as an operation in the case of a high step-down ratio (0.8 times or less) is different from an operation in the case of a low step-down ratio (0.8 times or more), it is difficult to make a step-down ratio variable while maintain a continuity of the step-down ratio.
As for configuration and method of the conventional DC/DC converter described in above FIG. 14 and FIG. 15, the number of a plurality of capacitors, C1 and C2, is required to increase, three or more in number, in order to achieve a step-up ratio of two times or more. For this reason, circuit configuration gets complicated. Further, as for the configuration of the conventional DC/DC converter described above, when a step-down ratio is low (0.8 to 1 times), there is a problem that an output current iL is turned into an interrupted current and is eventually interrupted. Moreover, as an operation in a case of a high step-down ratio (0.8 times or less) is different from an operation in a case of a low step-down ratio (0.8 times or more), it is impossible to make a step-down ratio variable in succession. Further, there is another problem that when a step-down ratio is considerably near to one time, a step-down operation is not efficient, similar to a step-down operation of a conventional L-type whose inductance is extremely low.